In particular, the present invention relates to a continuously variable transmission device of the type having planetary members in rolling contact with radially inner and outer races each comprising two axially spaced parts, with control means for selectively varying the axial separation of the two parts of one race and thus the radial position of the planetary members in rolling contact therewith. Such a transmission device may have means sensitive to the torque applied to one of two drive-transmitting members of the transmission (namely the input and output shafts) to determine the compensating variation in the separation of the two parts of the other race and thus the transmission ratio of the device, and also to vary the forces exchanged between the planets and the races normal to the interface between them. The rolling contact between the planetary members and the races is lubricated by means of a very thin film of lubricant. It is essential that this thin film of lubricant be present in order to prevent dry frictional contact between the members in relative motion, which would lead to premature wear, but also that such film should be extremely thin in order to avoid relative slippage.
It is an important design criterion that a transmission device should be most efficient in the transmission ratio most used, that is used for the greatest amount of the time. All transmission devices involve certain losses to friction, and thus heat, and the design resulting in greatest efficiency is usually applied to the so-called “top” transmission ratio, that is the ratio in which the output shaft rotates fastest for a given speed of rotation of the input shaft. In conventional incremental ratio gearboxes the greatest efficiency is usually achieved when the output shaft is travelling at the same speed as the input shaft to provide a 1:1 or “straight through” transmission ratio. There are, however, circumstances where the transmission ratio at greatest efficiency may be less than 1:1 and, correspondingly situations where a ratio of greater than 1:1 may be desirable.
In a continuously variable rolling contact transmission device of the type defined above the input to the device may be applied via the radially inner races and the output from the device taken from the planets via planet followers or a planet carrier, with the outer race constituting the stationary component. The high gear ratio is then achieved with the two components of the radially outer race located at their position of maximum spacing whilst the parts of the inner race are located as close to one another as possible so that the planets are, effectively, “squeezed” to their greatest radial position. Of course, it will be appreciated that the roles of input and output shaft can be reversed and, in the design in question, the roles of the three components, namely radially inner races, planets assembly, which includes planet followers and planet carriers, and radially outer races are all interchangeable so that any one of them may be held stationary and the other two used either as the input or the output member. It has been found, however, that a configuration as defined above with the outer race stationary has particular advantages from a constructional point of view.
One of the disadvantages arising from this configuration, however, if the planet is a ball, is that in order to obtain the highest ratio possible the patches where rolling contact takes place between the planets and the races are close to their end-of-range positions (closest to the axis of the ball in the case of the radially inner race and furthest from the axis of the ball in the case of the radially outer race). At the end-of-range positions the rolling of the planets over one or the other of the races involves a significant amount of “spin” at the contact patch between the planet and the race, and this generates considerable heat.